The present invention relates to a method for patterning a metal oxide thin film. More particularly, the present invention relates to a method for patterning a metal oxide thin film such as a ferroelectric film and an ITO film with accuracy of submicron order. The ferroelectric film is used in a storage cell or the like, and the ITO film is used in a liquid crystal display element or the like.
There are many kinds of metal oxide having various characteristic, such as, a high dielectric constant and consequent significant hysterisis of dielectric polarization, significant piezoelectric effect, significant pyroelectric effect, significant electro-optic effect, superconductivity or the like. The metal oxide has been utilized in every kinds of electronic parts such as a semiconductor storage cell, a piezoelectric element, a pyroelectric element, an electro-optic element, a liquid crystal display, an electroluminescent element, a photoelectric cell or the like.
Formation of a thin film of the metal oxide is carried out in a manner that the metal oxide is deposited on a substrate to form a film by sputtering (DC, RF, RF magnetron), evaporation, MOCVD (Metal Organic Chemical Vapor Deposition) or the like, thereafter the deposited film is burned to be crystalized. In that case, the deposition of film can be carried out under heating instead of separate deposition and burning steps. Fine processing of the thin film is carried out in a manner that a resist desirably patterned by lithography is applied on the thin film, thereafter there is performed wet etching with etchant of acid solution, physical dry etching with ion milling or plasma etching with HCl or CF.sub.4.
As another method for forming the metal oxide film, there is also employed a method wherein solution prepared by sol-gel method or MOD (Metal Organic Decomposition) method is applied by spin coating or dip coating on a substrate to form a film, and this film is burned to be crystalized. The film formation by the sol-gel method is disclosed for example in Japanese Unexamined Patent Publications No. 310969/1988, No. 108161/1989, No. 286922/1989, No. 287278/1989, No. 294522/1989 and No. 6335/1990. Fine processing of the metal oxide thin film formed by the disclosed methods is carried out, similar to the above, by wet etching, dry etching or plasma etching.
When the metal oxide thin film is burned (i.e. subjected to thermal treatment), a dense film is formed thereon so that the metal oxide thin film shows the above-mentioned characteristics. However, once the film is burned, it becomes very hard and therefore processing thereof becomes difficult. On the other hand, with recent tendency to much highly integrate electronic parts, for example semiconductor storage device wherein memory cell is integrated requires a fine processing with accuracy of submicron order not higher than 0.5 .mu.m.
If the patterning of the metal oxide thin film is carried out with conventional wet etching, a part to be left is narrowed by undercut because of isotropy of wet etching. Further the resist peels off under significant damage by strong acid or strong alkali and this causes deformation or losing of the pattern. This make it impossible to finely pattern with accuracy of submicron order. Thus, wet etching has some disadvantages.
Dry etching, for example ion milling, is good at finely patterning because dry etching is performed with ion (for example argon ion) beam. However, dry etching has disadvantages that it gives damage to an active lower substrate and it does not realize high "selective ratio". The "selective ratio" means a ratio of etching rates of the active substrate and the resist.
With respect to plasma etching, there is a problem that the etching rate is low and it is not suitable for mass production.
The present invention was made in view of the above-mentioned problems, and an object of the present invention is to provide a method for easily and finely processing a metal oxide thin film. By the method of the present invention, metal oxide thin film can be easily applied to highly integrated electronic parts such as a semiconductor storage device.